When it comes to memory retrieval, slow and steady wins the race. At least that is the finding from research at the University of Texas in Austin. Scientists have found a mechanism, which essentially compresses information needed for memory retrieval, imagination or planning and encodes it on a brain wave frequency that is separate from the one used for recording real-time experiences.
Featured in a recent edition of the journal Neuron, the findings are counter to what was previously believed about how memory is transmitted to and from the hippocampus.
Situated in the medial temporal lobe of the brain, the hippocampus is the centre of emotion, memory and the autonomic nervous system. It is crucial to the organisation of data, both for our short-term and long-term memories.
Like a conscientious editor, it brings all the memories gathered each day and tries to make something coherent out of it all. The hippocampus is assisted in this process by gamma rhythms – a type of brain wave that helps co-ordinate the activity of different brain cells acting together to carry out various functions.
Two types of gamma rhythms assist: low frequency (or slow) gamma and high frequency (or fast) gamma. It was widely understood that slow gamma rhythms carried less information than faster gamma, because there were fewer transmissions in any given cycle.
"The traditional viewpoint has been that one piece of information would be represented per gamma wave," explains Laura Colgin, assistant professor of neuroscience, at the Centre for Learning and Memory at the University of Texas (UT), Austin.
From this it was assumed that faster gamma waves were getting more data to the hippocampus than their slower counterparts simply because they were working faster. Upon closer scrutiny, however, the scientists found the opposite. Slower gamma is able to carry several pieces of data per cycle while fast gamma can carry only one unit of information.
Surf’s up
In any wave or cyclical pattern, there are different phases – the ocean’s waves advancing and receding, for example. In the case of memory transmission, there are phases where cells are active and other phases where cells get strongly inhibited, and are shut down.
“Think about a line of surfers at the beach,” says Colgin. “They’re all waiting to catch a wave but they’re coming in so fast, there’s room for only one surfer to catch each wave before it breaks.
“If the ocean swell reduces, the waves break more slowly so more surfers can catch the same wave. Because they come slowly, several surfers can surf a wave at the same time.
“Likewise, when the brain has an episode of slow gamma rhythms, a sequence of place cells should actually be coding a shorter path length within a certain amount of time. There are fewer slow gamma waves occurring within that time period, because it is at a lower frequency rhythm.” In short, units of information have more time to catch a ride on slow gamma.
“When the cell on the fast gamma cycle becomes active it may want to connect with another cell during earlier learning but there’s no time,” she says.
“There can only be that one piece of information represented in the cycle. “On the slower waves, however, the longer periods allow for cells to be more active. To put it another way, there is more time for a cell to activate another cell that it was aligned with earlier.”
Slow gamma mental compression is not dissimilar to the process of compressing files on a computer. However, like digital compression, when a previously compressed mental memory is replayed, the representation will likely have far less detail than the original source material.
Health implications
The findings have implications for medicine as well as for criminal justice or any situation where memory reliability is being scrutinised. It is understood that gamma oscillations are disrupted in different disorders such as schizophrenia, Alzheimer’s and autism.
Colgin and the research team believe the findings could help explain why people with schizophrenia, who are experiencing disrupted gamma rhythms, have difficulty distinguishing between imagined and real experiences.
“Maybe they are transmitting their own imagined thoughts on the wrong frequency, the one usually reserved for things that are really happening.”
The UT research team has started follow-on research based on a mouse model with Alzheimer’s disease. “Very preliminary research has shown some disruptions in the selection of one type of gamma and not in the other that could possibly be significant,” says Colgin.
“At this point, it is just speculation, but perhaps the cells in the brain of someone with Alzheimer’s may not be co-ordinated in the same way during slow gamma frequency and this could possibly relate to why there is a problem with memory retrieval.”
